This invention relates to aluminum or aluminum alloy parts and their use in applications requiring wear resistance and more particularly relates to ceramic coated aluminum oxide articles having improved wear resistance and to their method of manufacture.
Because of weight reduction and corrosion resistance, the use of aluminum and its alloys in numerous applications is desirable. Unfortunately aluminum and its alloys are not suitable in such applications where high wear resistance is needed.
Aluminum products have been known that have oxidized surfaces that increase corrosion resistance, and to some extent wear resistance, through a process known as anodization. In such a process aluminum or its alloys are immersed in an acidic electrolyte and subjected to a DC current as an anode to form a corrosion inhibiting aluminum oxide layer. Such anodized layers may also somewhat increase wear resistance, especially in the case of “hardcoat” anodizing that can provide oxide thicknesses of as much as about 0.01 inch (250 μm). The increased wear resistance is believed to be at least partly due to the increased thickness but the wear resistance is still not nearly as good as needed or desired in machine parts.
More recently, e.g. as described in U.S. Pat. Nos. 5,720,866 and 6,365,028, incorporated herein by reference as background art, a process has been described that uses alternating current, instead of a direct current and an alkaline electrolyte, instead of an acidic electrolyte to obtain a coating on aluminum and its alloys. The described process is complex generally requiring an at least three component electrolyte containing a relatively large amount of a pyrophosphate and requiring a high starting current density.
Using the process described in this patent, articles can be obtained that have greatly improved wear resistance as compared with anodized products. Such products have been made by Keronite International, Ltd. of the United Kingdom. Wear resistance increases of as much as twenty times that of aluminum or aluminum alloy alone have been reported.
In any case a wear resistance increase of twenty times over untreated aluminum alloy is significant but still not as good as desired.